LM2451

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X IK OBSOLETE LM2451 www.ti.com SNOSAN7I –AUGUST 2005–REVISED APRIL 2013 LM2451220VMonolithicTripleChannel12MHzDC CoupledCRTDTVDriver Check forSamples: LM2451 1FEATURES DESCRIPTION The LM2451 is a triplechannel high voltageDC 2• 12 MHz Bandwidth at130VP-P coupledCRT drivercircuitdesignedforuse inDTV• 100V Black LevelAdjustment Range Using 0V applications.The IC contains three high inputto5V Input impedance,wide band amplifierswhich directlydrive

• CurrentOutput forIK Feedback Systems the RGB cathodesof a CRT. Each amplifierhas a summing inputwhere the DC levelof the outputis• Greaterthan 130VP-P Output Swing Capability controlledby a low voltage DC input voltage.• 0V to5V InputVoltageRange Normallythe DC inputvoltageisfrom a DAC. Each
• Stablewith0 pF–20 pF CapacitiveLoads and channel has itsgain internallyset to −54 and can InductivePeaking Networks driveCRT capacitiveloadsas wellas resistiveloads presentin other applications,limitedonly by the• Convenient TO-220 Staggered Thin Lead package'spower dissipation.Package Style The IC is packaged in a 15-leadTO-220 molded plasticpower package designedspecificallyto meetAPPLICATIONS high voltagespacingrequirements.See THERMAL• DC Coupled DTV ApplicationsUsing the480p CONSIDERATIONS section.Format as wellas Standard NTSC and PAL Formats. Connection Diagram Figure1. TO-220 (Top View) See Package Number NDN Pleasebe aware thatan importantnoticeconcerningavailability,standardwarranty,and use incriticalapplicationsof Texas Instrumentssemiconductorproductsand disclaimerstheretoappearsattheend ofthisdatasheet. 2Alltrademarksarethepropertyoftheirrespectiveowners. PRODUCTION DATA informationiscurrentas ofpublicationdate. Copyright© 2005–2013,Texas InstrumentsIncorporatedProductsconform to specificationsper the terms of the Texas Instrumentsstandardwarranty.Productionprocessingdoes not necessarilyincludetestingofallparameters.

13, 14, 15 IK 9,10,11 VIN VDAC 2,4,6 R13 VCC R10R2R9 R6 R3 Rb R12 R11 1,3,5 Q1 Q7 VBB OBSOLETE LM2451 SNOSAN7I –AUGUST 2005–REVISED APRIL 2013 www.ti.com Schematic Diagram Figure2. SimplifiedSchematic Diagram (One Channel) These deviceshave limitedbuilt-inESD protection.The leadsshouldbe shortedtogetherorthedeviceplacedinconductivefoam duringstorageorhandlingtopreventelectrostaticdamage totheMOS gates. AbsoluteMaximum Ratings(1)(2)(3) SupplyVoltage(VCC ) +250V BiasVoltage(VBB ) +16V InputVoltage(VIN) −0.5VtoVBB +0.5V StorageTemperatureRange (TSTG ) −65°C to+150°C Lead Temperature(Soldering,<10 sec.) 300°C ESD Tolerance, Human Body Model 2 kV Machine Model 200V JunctionTemperature 150°C θJC (typ) 4.0°C/W (1) Allvoltagesaremeasured withrespecttoGND, unlessotherwisespecified. (2) AbsoluteMaximum Ratingsindicatelimitsbeyond whichdamage tothedevicemay occur. (3) IfMilitary/Aerospacespecifieddevicesarerequired,pleasecontacttheTexas InstrumentsSalesOffice/Distributorsforavailabilityand specifications. OperatingRatings(1) VCC +100V to+230V VBB +7V to+13V VIN +0V to+5V VOUT +40V to+215V Case Temperature(10W max power) 110°C Do notoperatethepartwithouta heatsinkand thermalgrease.Heat sinkmust have a thermalresistanceunder5.0°C/W.(2) (1) Operatingratingsindicateconditionsforwhichthedeviceisfunctional,butdo notensurespecificperformancelimits.Forensured specificationsand testconditions,see theElectricalCharacteristics.Datasheetmin/max specificationlimitsarespecifiedby design,test, orstatisticalanalysis.The ensuredspecificationsapplyonlyforthetestconditionslisted.Some performancecharacteristicsmay change when thedeviceisnotoperatedunderthelistedtestconditions. (2) Runningthe1 MHz to30 MHz testpatternat1080ithispartwilldissipateapproximately10 W. Thisisthecommonly acceptedtest patternthatisrepresentativeoftheworstcase highfrequencycontentfornormaltelevisionviewing.Thisisthepatternused toestimate theworstcase power dissipationoftheLM2451 initsnormalapplication.Itisrecommended touse a heatsinkwitha thermalresistance of5.0°C/W orbetter.

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+220V+12V C L = 10 pF 50: Output to 50: Scope 10 PF 0.1 PF 1,3,5 2451 13,14,15 47 PF 0.1 PF VADJ 50: 1000: 0.47 PF 0.01 PF VIN 9,10,11 50: Test Point 4990: 4990: 0.1 PF Pulse Generator Input VDAC 2,4,6 8 IK Note: VDAC < 0.5V OBSOLETE LM2451 www.ti.com SNOSAN7I –AUGUST 2005–REVISED APRIL 2013 ElectricalCharacteristics (See Figure3 forTestCircuit).Unlessotherwisenoted:VCC = +220V, VBB = +12V, VDAC = +0.5V,C L = 10 pF,TC = 50°C. DC Tests:VIN = +2.7VDC .AC Tests:Output= 130VPP (60V – 190V) at1 MHz. LM2451 Symbol Parameter Conditions Units Min Typ Max ICC SupplyCurrent No InputSignal,No VideoInput,No 10 18 25 mAOutputLoad IBB BiasCurrent 18 26 34 mA VOUT, 1 DC OutputVoltage No AC InputSignal,VIN = 2.7VDC 122 127 132 VDC VOUT, 2 DC OutputVoltage No AC InputSignal,VIN = 1.2VDC 200 205 210 VDC VOUT, 3 DC OutputVoltage No AC InputSignal,VIN = 1.2VDC ,VDAC = 192 198 204 VDC1.2VDC VOUT, 4 DC OutputVoltage No AC InputSignal,VIN = 1.2VDC ,VDAC = 154 160 166 VDC2.7VDC AV DC VoltageGain No AC InputSignal −51 −54 −57 V/V ADAC DAC InputDC VoltageGain No AC InputSignal −23 −26 −29 V/V ΔAV Gain Matching See (1),No AC InputSignal 1.0 dB LE LinearityError See (1)(2),No AC InputSignal 8 % tr RiseTime See (3),10% to90% 31 ns +OS Overshoot 10 % tf FallTime See (3),90% to10% 34 ns −OS Overshoot See (3) 3 % BW L LargeSignalBandwidth VOUT AC = 130 VP-P,VOUT DC = 125 V 12 MHz BW M Medium SignalBandwidth VOUT AC = 100 VP-P,VOUT DC = 125 V 14 MHz BW S SmallSignalBandwidth VOUT AC = 60 VP-P,VOUT DC = 125 V 16 MHz IkERROR CurrentOutputError OutputCurrent= 0 µA to200 µA −52 0 52 μA ΔIkERROR CurrentOutputDifference OutputCurrent= 0 µA to200 µA 0 NA 32 μABetween Channels (1) CalculatedvaluefromVoltageGain teston each channel. (2) LinearityErroristhevariationinDC gainfromVIN = 1.10VtoVIN = 4.30V. (3) Inputfromsignalgenerator:tr,tf< 10 ns. AC TestCircuit Note:10 pF loadincludesparasiticcapacitance. Figure3. TestCircuit(One Channel) Figure3 shows a typicaltestcircuitforevaluationoftheLM2451. Thiscircuitisdesignedtoallowtestingofthe LM2451 ina 50Ω environmentwithoutthe use of an expensiveFET probe.The two 4990Ω resistorsform a 400:1dividerwiththe50Ω resistorand theoscilloscope.A testpointisincludedforeasy use ofan oscilloscope probe.The compensationcapacitorisused tocompensate thenetworktoachieveflatfrequencyresponse. Copyright© 2005–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 3 ProductFolderLinks:LM2451

SNOSAN7I –AUGUST 2005–REVISED APRIL 2013 www.ti.com TypicalPerformance Characteristics (VCC = +220VDC ,VBB = +12VDC ,C L = 10 pF,VOUT = 130VPP (60V – 190V),TC = 50°C, TestCircuit— Figure3 unlessotherwise specified) Figure4.VOUT vs VIN Figure5. LM2451 Pulse Response Figure6. Bandwidth Figure7.Speed vs Load Capacitance Figure8.Speed vs Offset Figure9.Speed vs Case Temperature

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www.ti.com SNOSAN7I –AUGUST 2005–REVISED APRIL 2013 TypicalPerformance Characteristics(continued) (VCC = +220VDC ,VBB = +12VDC ,C L = 10 pF,VOUT = 130VPP (60V – 190V),TC = 50°C, TestCircuit— Figure3 unlessotherwise specified) Figure10. Power Dissipationvs Frequency Figure11. Safe OperatingArea Figure12. LM2451 Cathode Response Copyright© 2005–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 5 ProductFolderLinks:LM2451

SNOSAN7I –AUGUST 2005–REVISED APRIL 2013 www.ti.com THEORY OF OPERATION The LM2451 isa highvoltagemonolithicthreechannelCRT driversuitableforDTV applications.The LM2451 operateswith220V and 12V power supplies.The partishoused ina 15-leadTO-220 molded plasticpower package withthinleadsforimprovedmetal-to-metalspacing. The circuitdiagramoftheLM2451 isshown inFigure2.The PNP emitterfollower,Q5, providesinputbuffering. Q1 and Q2 form a fixedgaincascode amplifierwithresistorsR1 and R2 settingthegainat−54.An additional cascode amplifierisformed by Q7 and Q2. Gain of thisstage isset to — 25 by resistorsR1 and R10. Q8 providestheinputbufferingforthisinput.Q2 now becomes thesumming pointforbothVIN and VDAC .Emitter followersQ3 and Q4 isolatethehighoutputimpedance ofthecascode stagefrom thecapacitanceoftheCRT cathode,which decreasesthe sensitivityof the deviceto loadcapacitance.Q6 providesbiasingto the output emitterfollowerstagetoreducecrossoverdistortionatlowsignallevels. Figure3 shows a typicaltestcircuitforevaluationoftheLM2451. Thiscircuitisdesignedtoallowtestingofthe LM2451 ina 50Ω environmentwithouttheuse ofan expensiveFET probe.Inthistestcircuit,thetwo 4.99kΩ resistorsforma 400:1wideband,low capacitanceprobewhen connectedtoa 50Ω coaxialcableand a 50Ω load (suchas a 50Ω oscilloscopeinput).The inputsignalfromthegeneratorisAC coupledtothevideoinputsofthe LM2451. ApplicationHints INTRODUCTION Texas Instrumentsiscommittedto provideapplicationinformationthatassistsour customersinobtainingthe best performancepossiblefrom our products.The followinginformationisprovidedin orderto supportthis commitment.The readershouldbe aware thattheoptimizationofperformancewas done usinga specificprinted circuitboard.Variationsinperformancecan be realizeddue tophysicalchanges intheprintedcircuitboardand theapplication.Therefore,thedesignershouldknow thatcomponent valuechanges may be requiredinorderto optimizeperformanceina givenapplication.The valuesshown inthisdocument can be used as a startingpoint forevaluationpurposes.When workingwithhighbandwidthcircuits,good layoutpracticesare alsocriticalto achievingmaximum performance. IMPORTANT INFORMATION The LM2451 performanceistargetedfortheHDTV market.The applicationcircuitsshown inthisdocument to optimizeperformanceand to protectagainstdamage from CRT arc over are designed specificallyforthe LM2451. Ifanothermember oftheLM245X familyisused,pleaserefertoitsdatasheet. POWER SUPPLY BYPASS Since the LM2451 is a wide bandwidth amplifier,proper power supply bypassingis criticalforoptimum performance.Improperpower supplybypassingcan resultin largeovershoot,ringingor oscillation.0.1 µF capacitorsshouldbe connectedfrom the supplypins,VCC and VBB , to ground,as closeto the LM2451 as is practical.Additionally,a 22 µF or largerelectrolyticcapacitorshouldbe connectedfrom both supplypinsto groundreasonablyclosetotheLM2451. ARC PROTECTION DuringnormalCRT operation,internalarcingmay occasionallyoccur.Thisfast,highvoltage,high-energypulse can damage theLM2451 outputstage.The applicationcircuitshown inFigure13 isdesignedtohelpclamp the voltageattheoutputoftheLM2451 toa safelevel.The clamp diodes,D1 and D2, shouldhave a fasttransient response,high peak currentrating,low seriesimpedance and low shunt capacitance.1SS83 or equivalent diodes are recommended. D1 and D2 should have short,low impedance connectionsto VCC and ground respectively.The cathodeofD1 shouldbe locatedveryclosetoa separatelydecoupledbypass capacitor(C3 in Figure13).The ground connectionof D2 and the decouplingcapacitorshouldbe verycloseto the LM2451 ground.ThiswillsignificantlyreducethehighfrequencyvoltagetransientsthattheLM2451 would be subjected toduringan arcovercondition.ResistorR2 limitsthearcovercurrentthatisseen by thediodeswhileR1 limits thecurrentintotheLM2451 as wellas thevoltagestressattheoutputsofthedevice.R2 shouldbe a ½ W solid carbontyperesistor.R1 can be a ¼ W metalorcarbonfilmtyperesistor.HavinglargevalueresistorsforR1 and R2 wouldbe desirable,butthishas theeffectofincreasingriseand falltimes.InductorL1 iscriticaltoreducethe

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R TH = 110oC - 60oC 10W = 5.0oC/W 2451 +220V+12V INPUT 220:L1 D1 CATHODE C2C1 1/2W6.8 PH510: OBSOLETE LM2451 www.ti.com SNOSAN7I –AUGUST 2005–REVISED APRIL 2013 initialhighfrequencyvoltagelevelsthatthe LM2451 would be subjectedto beforethe clamp diodeshave a chance tobecame activated.The inductorwillnotonlyhelpprotectthedevicebutitwillalsohelpminimizerise and falltimesas wellas minimizeEMI. For properarc protection,itisimportantto not omitany of the arc protectioncomponents shown inFigure13.NOTE: Do notuse sparkgaps withtheLM2451 when designingDC coupledsystem.Ifa sparkgap istriggereditshortsouttheoutputoftheLM2451 and willdestroythepart. Figure13. One Channel oftheLM2451 withtheRecommended ApplicationCircuit EFFECT OF LOAD CAPACITANCE Figure7 shows the effectof increasedloadcapacitanceon the speed of the device.Thisdemonstratesthe importanceofknowingtheloadcapacitanceintheapplication.Increasingtheloadcapacitancefrom10 pF to20 pF adds about 14 ns to the risetimeand about 18 ns to the falltime.Itisveryimportantto keep the board capacitanceas lowas possibletomaximizethespeed ofthedriver. EFFECT OF OFFSET Figure8 shows the variationinriseand falltimeswhen the blacklevelof the deviceisvariedfrom 180V to 200VDC .The risetimeincreasesonlyabout3 ns as theoffsetisincreasedinvoltageand thefalltimedecreases by about7 ns withthesame offsetadjustment. THERMAL CONSIDERATIONS Figure9 shows the performanceof the LM2451 in the testcircuitshown in Figure3 as a functionof case temperature.The figureshows thattherisetimeoftheLM2451 increasesby about7ns as thecase temperature increasesfrom30°C to110°C. Over thesame case temperaturerangethefalltimeincreasedby about9 ns. Figure10 shows themaximum power dissipationoftheLM2451 vs.Frequencywhen allthreechannelsofthe devicearedrivingintoa 10 pF loadwitha 130VP-P alternatingone pixelon,one pixeloff.Note thatthefrequency giveninFigure10 ishalfofthepixelfrequency.The graphassumes an 80% activetime(deviceoperatingatthe specifiedfrequency),whichistypicalina TV application.The other20% ofthetimethedeviceisassumed tobe sittingattheblacklevel(190V inthiscase).A TV picturewillnothave frequencycontentoverthewhole picture exceeding15 MHz. Itisimportanttoestablishtheworstcase conditionunder normalviewingtogivea realistic worst-casepower dissipationfortheLM2451. One testisa 1 to30 MHz sinewave sweep overtheactiveline. This would givea slightlylowerpower than takingthe average of the power between 1 and 30 MHz. This average is 10.4 W. A sine wave willdissipateslightlylesspower, probablyabout an even 10W of power dissipation.Allof thisinformationis criticalforthe designerto establishthe heat sinkrequirementforhis application.The designershouldnote thatifthe loadcapacitanceisincreasedthe AC component of the total power dissipationwillalsoincrease. The LM2451 case temperaturemust be maintainedbelow 110°C giventhemaximum power dissipationestimate of10W. Ifthemaximum expectedambienttemperatureis60°C and themaximum power dissipationis10W then a maximum heatsinkthermalresistancecan be calculated: (1) Thisexample assumes a capacitiveloadof10 pF and no resistiveload.The designershouldnotethatiftheload capacitanceisincreasedtheAC component ofthetotalpower dissipationwillalsoincrease.Iftheworstcase power dissipationisnot 10W pleasereferto Figure11 forthe maximum case temperatureforthe expected maximum power dissipation. Copyright© 2005–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 7 ProductFolderLinks:LM2451

SNOSAN7I –AUGUST 2005–REVISED APRIL 2013 www.ti.com OPTIMIZING TRANSIENT RESPONSE Referringto Figure13, thereare threecomponents (R1, R2 and L1) thatcan be adjustedto optimizethe transientresponseoftheapplicationcircuit.IncreasingthevaluesofR1 and R2 willslowthecircuitdown while decreasingovershoot.IncreasingthevalueofL1 willspeed up thecircuitas wellas increaseovershoot.Itisvery importantto use inductorswithvery high self-resonantfrequencies,preferablyabove 300 MHz. Ferritecore inductorsfromJ.W.MillerMagnetics(part# 78FR--K)were used foroptimizingtheperformanceofthedevicein theapplicationboard.The valuesshown inFigure13 can be used as a good startingpointfortheevaluationof theLM2451. Usinga variableresistorforR1 willsimplifyfindingthevalueneeded foroptimum performanceina givenapplication.Once theoptimum valueisdeterminedthevariableresistorcan be replacedwitha fixedvalue. Due toarcoverconsiderationsitisrecommended thatthevaluesshown inFigure13 notbe changed by a large amount. Figure12 shows thetypicalcathodepulseresponsewithan outputswingof130VPP insidea modifiedproduction TV setusingtheLM1237 pre-amp. PC BOARD LAYOUT CONSIDERATIONS For optimum performance,an adequateground plane,isolationbetween channels,good supplybypassingand minimizingunwanted feedbackarenecessary.Also,thelengthofthesignaltracesfromthesignalinputstothe LM2451 and fromtheLM2451 totheCRT cathodeshouldbe as shortas possible.The followingreferencesare recommended: Ott,Henry W.,“NoiseReductionTechniquesinElectronicSystems”,John Wiley& Sons,New York,1976. “VideoAmplifierDesignforComputer Monitors”,ApplicationNote 1013. Pease,RobertA.,“TroubleshootingAnalogCircuits”,Butterworth-Heinemann,1991. Because ofitshighsmallsignalbandwidth,thepartmay oscillateina TV iffeedbackoccursaround thevideo channelthroughthechassiswiring.To preventthis,leadstothevideoamplifierinputcircuitshouldbe shielded, and inputcircuitwiringshouldbe spaced as faras possiblefromoutputcircuitwiring. TYPICAL APPLICATION A typicalapplicationoftheLM2451 isshown inFigure14.Used inconjunctionwitha pre-amp witha 1.2V black leveloutputno buffertransistorsarerequiredtoobtainthecorrectblacklevelatthecathodes.Ifthepre-amp has a blacklevelcloserto2V, thenan NPN transistorshouldbe used todropthevideoblacklevelvoltagecloserto 1.2V.When usingonlyone NPN transistoras an emitterfollower,a jumperneeds tobe added ineach channel. Inthered channela jumperneeds tobe added between C7 and R25. With justone transistorneitherofthese components wouldbe installed. InadditiontothevideoinputsaretheDAC inputs.These inputsareused tovarytheLM2451 outputblacklevel by a DAC. inthe pastwhen a driverwas used witha CMOS AVP therewas not enough range on the video outputtovarytheblacklevel.A clamp circuithad tobe used inconjunctionwiththeAVP and thedriver.The DAC inputsoftheLM2451 aredriveninthesame way theclamp circuithad been driven,eliminatingtheneed for a clamp circuit.Figure4 shows the variationinthe blacklevelas the DAC inputvoltageischanged.Thisis shown forbothVIN = 1.2Vand VIN = 2.1V. The neck boardinFigure14 has two transistorsineach channelenablingthisboardtowork withpre-ampswith a blackleveloutputas highas 2.5V.Each transistorstagehas a gainof−1.Thissetupstillgivesthetwo diode drop atthedriverinput;however,now additionalpeakingcan be done on thevideosignalbeforereachingthe driverinputs.Some popularAVPs do have a blacklevelof 2.5V.For lowerblacklevelseitherone or both transistorswouldnotbe used. ItisimportantthattheTV designeruse component valuesforthedriveroutputstageclosetothevaluesshown inFigure14. These valueshave been selectedto protectthe LM2451 from arc over.Diodes D1,D8, D9, and D13 –D15 must alsobe used forproperarcoverprotection.The demonstrationboard can be used toevaluate theLM2451 ina TV.

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www.ti.com SNOSAN7I –AUGUST 2005–REVISED APRIL 2013 DEMONSTRATION BOARD Figure15 shows the routingand component placement on the LM2451 demonstrationboard.This board providesa good example of a layoutthatcan be used as a guideforfuturelayouts.Note the locationof the followingcomponents:

• C26 — VCC bypasscapacitor,locatedveryclosetopin12 and groundpins
• C27 — VBB bypasscapacitor,locatedclosetopin7 and ground
• C28, C30, C33 — VCC bypass capacitors,near LM2451 and VCC clamp diodes.Very importantforarc protection. The routingof the LM2451 outputsto the CRT isverycriticalto achievingoptimum performance.Figure16 shows theroutingand component placementfrom pin13 (VOUT3 ) oftheLM2451 tothebluecathode.Note that thecomponents areplacedso thattheyalmostlineup fromtheoutputpinoftheLM2451 tothebluecathodepin of the CRT connector.Thisisdone to minimizethe lengthof the videopath between thesetwo components. Note alsothatD1, D8 and R36 areplacedtominimizethesizeofthevideonodes thattheyareattachedto.This minimizesparasiticcapacitanceinthevideopathand alsoenhances theeffectivenessoftheprotectiondiodes. The anode ofprotectiondiodeD1 isconnecteddirectlytoa sectionoftheground planethathas a shortand directpathtotheCRT groundand theLM2451 groundpins.The cathodeofD8 isconnectedtoVCC verycloseto decouplingcapacitorC28 whichisconnectedtothesame areaofthegroundtraceas D1. The diodeplacement and routingisveryimportantforminimizingthevoltagestresson theLM2451 duringan arcoverevent. Thisdemonstrationboarduses largePCB holestoaccommodate socketpins,whichfunctiontoallowformultiple insertionsof the LM2451 in a convenientmanner. To benefitfrom the enhanced LM2451 package withthin leads,the deviceshouldbe securedinsmallPCB holesto optimizethe metal-to-metalspacingbetween the leads. CURRENT OUTPUT FOR IK FEEDBACK SYSTEMS The LM2451 can be used inDTV applicationsthatuse an IK feedbacksystem.Figure14 shows an example of an interfacecircuitused tofeedback theIKoutputofLM2451 toa preamplifierwithan ac coupledIKinput. This feedback system consistsof the preamp, LM2451, and interfacecircuit,forminga closed loop to automaticallyadjusttheblacklevelofthedrivesignalstothecutoffpointoftheRGB cathodes.Followingisa descriptionoftheinterfacecircuitoperationused forAVPs thathave a voltageinputfortheirIKsense input. The outputatpin8 oftheLM2451 isfilteredofhighfrequencynoiseby C14. D7 isused tolimitthepeak voltage atpin8.Withoutthisclamp diodethevoltagewould easilyexceed 12V duringactivevideowhen thecathode currentsaremuch greaterthanthesmallcurrentsbeingdetectedduringverticalblanking.Exceeding12V could damage Q4 and resultinimproperoperationofthedriver. R35 isessentialtoconverttheIK currenttovoltage.Choosing thevalueofR35 setsthegainofthefeedback voltage,and consequently,theoperatingpointofthetube.Once a stableoperatingpointisestablished,thispoint can be fine-tunedusingtheadjustmentrangeofthefeedbacksystemorstandardpreamp controls.Changingthe valueofR35 willchange thecutoffvoltageatthecathode.A smallervalueofR35 requiresmore IK currentto maintainthefeedbackloop.The cutoffvoltagesetatthecathodewillbe lowertoadjusttothehigherIK current. Thisadditionalcurrentmust come from thecathode;therefore,thecathodevoltageissetlowertomeet higher currentrequirement.A highervalueof R35 willdo the opposite,raisingthe cathodevoltagebecause lessIK currentisneeded tomaintainthesame voltageatR35. The emitterfollower,Q7, isolatesR35 fromtheinputimpedance ofthepreamp.R21 and R39 biastheemitterof Q7 tolimitthemaximum voltagetothepreamp. These resistorvaluesshouldbe chosen tolimitthemaximum voltageattheemitterand protectthepreamp from any largevoltagesthatwould otherwiseoccurduringactive video.C9 isused to AC couplethe IK signalto the preamp. The advantageof AC couplingisthatany DC component (leakagecurrentfromthedriver)oftheIKsignalisnotdetectedby theIKsense inputofthepreamp. Some AVPs do have a directcurrentinputfortheirIK sense input.For interfacingto these AVPs the only components tobe used intheIK sense sectionareD7 and R41. To completethesignalpatha jumpermust be used toreplaceR34, C9 and thebase-emitterjunctionofQ7. C14 can stillbe used forhighfrequencyfiltering. Copyright© 2005–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 9 ProductFolderLinks:LM2451

SNOSAN7I –AUGUST 2005–REVISED APRIL 2013 www.ti.com Figure14. LM2451 DTV ApplicationsCircuit

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www.ti.com SNOSAN7I –AUGUST 2005–REVISED APRIL 2013 Figure15. LM2451 DTV DemonstrationBoard Layout Copyright© 2005–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 11 ProductFolderLinks:LM2451

SNOSAN7I –AUGUST 2005–REVISED APRIL 2013 www.ti.com Figure16. Trace Routing and Component Placement forBlue Channel Output

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REVISION HISTORY

Changes from RevisionH (April2013)toRevisionI Page Copyright© 2005–2013,Texas InstrumentsIncorporated SubmitDocumentationFeedback 13 ProductFolderLinks:LM2451

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